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Controls on Shelf-Margin Growth and Reservoir Development in an Active, Supply-Dominated and Greenhouse Early Cretaceous Margin Across Timescales

Abstract

Climatic, eustatic and tectonic forces have a significant impact on shelf-to-basin sediment transfer mechanisms and reservoir distribution. The key challenge for geoscientists is to understand how these forcing parameters are interacting across timescales and impact the short-term (4th-5th orders) to long-term (3rd order) controls on reservoir distribution.

This study focuses on the Lower Barrow Group (LBG), a shelf margin (~100-500 m relief) developed in syn-rift conditions during the Early Cretaceous (North West Shelf, Australia). Seismic interpretation of high-quality 3D seismic data allowed the identification of 73 high-resolution clinothems (~63,000 yrs duration each). Thanks to observation-based sequence stratigraphic techniques, 3rd to 5th order stratigraphic sequences were identified.

Rift-related tectonic processes were key forcing parameters on the coupled subsidence in the basin and uplift in the hinterland that directly impacted the variability of the rates of accommodation creation and sediment supply at the 3rd order. Overall, supply-dominated conditions were promoted by the presence of a proximal relief.

High rates of sediment supply and the presence of high-frequency sea-level fluctuations during the “cooler” greenhouse conditions of the Early Cretaceous also promoted the: (1) brief shoreline retreat during transgressive half cycles (the turning point between transgression and regression is reached quickly); and (2) rapid transit of the shoreline to the shelf edge during regressive half-cycles.

At the 5th order, the redistribution of sediments at the shelf edge was influenced locally by the: (1) hydrodynamic regime, where fluvial channels promoted the delivery of sand to the slope and basin; (2) presence of longshore currents drifting the sandy fraction of the sediments supplied by rivers to accrete beach ridges; and (3) presence of bottom currents reworking the muddy fraction of the sediments supplied by rivers to develop mud belts.

Overall, the high sediment discharge at the shelf edge at the 3rd to 5th orders promoted the frequent delivery of large volumes of sand to deep-water areas, as the deltaic shorelines and fluvial feeders could remain located at the shelf edge during longer periods of time. Therefore, sediment supply was a key driver of the LBG shelf-margin growth and sand distribution under local (process regime, currents), regional (tectonic setting) and global (climate, eustasy) forcing parameters interacting across timescales.